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Hematology is the study of blood, blood-forming tissue, and disorders of blood; first step toward knowing composition of blood came in 17th century when red corpuscles were observed using primitive microscope; later role of fibrin in clotting found and bone marrow as site of blood-cell formation recognized; 20th century opened with discovery of blood groups, making transfusions without serious ill effects possible; 1920s began systematic investigation of role of food substances in production of red blood cells; research since World War II has made use of electron microscopy and advances in biochemistry.